No. 6 Crown lightening to reduce the risk of tree topple
NEW ZEALAND FARM FORESTRY ASSOCIATION INFORMATION LEAFLET
Introduction
Juvenile instability or ‘toppling’, is the leaning of young trees (2 to
3 years old) due to failure of root anchorage. It is particularly
likely to occur on heavy soils, especially after heavy rain. Trees
affected by topple are often not uprooted, but continue to grow,
resulting in sweep and stem sinuosity as the tree struggles back to the
vertical.
Toppling has serious economic implications for forest and woodlot
growers throughout New Zealand. It results in more stems having butt
sweep and increased quantities of compression wood, so reducing
potential volume recovery of valuable clearwood. Loss of trees due to
topple also reduces selection ratios at time of thinning, reducing the
possibility of achieving an acceptable final crop stocking. A further
cost is the increased
susceptibility of older trees to windthrow due to poor root
architecture.
Crown lightening, reducing a tree’s ‘sail area’ by shortening the
branches of rising 2 year old trees, is one method of possibly reducing
the incidence of toppling. While the application of crown lightening is
becoming increasinglym common, the name used to describe the technique,
and the method of application are extremely varied.
Ian Barton in the November 1995 New Zealand Tree Grower documented
results of a trial in which three levels of crown lightening were
applied to Cupressus spp. This trial found that sail pruning had no
effect on height increment. Toppling was identified only in the
C.lusitanica control and ‘even
prune’ trees. An ‘even prune’ involved
removing every third branch starting from the top.
Jeff Tombleson, and James Turner of Forest Research have installed a
number of trials in 2 year old radiata pine seedlings evaluating a
crown lightening technique which aims to remove approximately one third
of the crown area, by shortening of all branches to form a cylinder.
Geoff and Gill Brann have applied crown lightening to two year old
radiata pine seedlings growing on fertile farm sites which experience
exceptional growth rates. Their crown lightening operation involves tip
pruning all branches to remove at least 50% of foliage, and cost 15 to
20 cents/tree. They began ‘sail pruning’ 15 to 20 years ago. One
problem they have experienced has been the increased number of
epicormic shoots which add an extra cost at first pruning.
City Forests Ltd in Dunedin, have been applying crown lightening on 2
to 3 year old radiata pine seedlings grown on ex-farm sites. The
combined form pruning and crown lightening operation involves removal
of multiple leaders, and shortening branches where possible to 30 to
50cm. This treatment costs approx. 20 cents/tree. The crown lightening
component of the treatment which was used on 300ha over the last 2
years was considered of limited value given the cost of treatment and
the levels of topple experienced in treated stands.
The trial
Results
Implications for
growers
The trial
The Tree People Ltd have been using a crown lightening technique termed
‘windproofing’ for several years. Their
 |
| A two-year-old
radiata pine seedling prior to crown lightening (left) and following
crown lightening (right). |
treatment involves shortening
the branches in whorls above half the tree height in two year old
trees. Each branch is shortened to half the mean length of the branches
in the whorl. Any branches in the lower half of the tree that extend
into the top half of the tree are cut at half height. The treatment
aims to reduce the sail area at the point of greatest leverage on the
tree during the critical period of susceptibility through the following
winter. Shortening, rather than removing branches, allows them to
continue to grow and also shade the stem to minimise epicormic growth.
Results indicated that this treatment helped reduce the incidence of
topple, but the effect on growth and form was unknown.
To test this, a trial was established 30km west of Whangarei on a
fertile farm site. The soil is a clay loam and is in good quality
pasture of rye grass, brown top and paspalum with some scattered
patches of blackberry. GF17 bare-rooted seedlings were established in
August 1994 at 800 stems/ha using a 2-cut planting method ensuring the
roots were not distorted at the time of planting. Trees were release
sprayed with Galant and Gardoprim in
September 1994.
Crown lightening was carried out over part of the area on 20 June when
the trees were rising two years old. Two days later an easterly storm
with winds gusting up to 95km/hr resulted in considerable toppling
within the trial area, providing an opportunity to evaluate the
effectiveness of the crown lightening treatment.
Percentage incidence and
degree of topple
|
Control (%) |
Windproofed (%) |
| No topple |
50 |
100 |
| Topple >0 <15 degrees |
16 |
0 |
| Topple >15 degrees |
34 |
0 |
Results
Growth.
The wind-proofed trees had slightly smaller diameters than
the
untreated trees 12 months after treatment, but by age 4 there was no
significant difference. There was no significant difference between
treatments in tree height at  either one or
two years following
treatment.
Topple
and lean. There is a significant difference in incidence
between
the control and wind-proofed treatments. None of the wind-proofed trees
toppled, but 50% of the untreated trees had an average lean of 30º.
Straightness.
Trees that toppled continued to grow, but their stems
curved as they struggled back to the vertical, giving them the
appearance of hockey sticks. Topple in the untreated trees resulted in
significantly poor stem straightness: a deviation of 9.5mm/m compared
with 2.4mm/m in the treated trees.
Sweep.
Two years after treatment none of the treated trees had sweep of
more than 10mm/m, but 20% of the untreated trees did.
Implications
for growers
Toppling is a random event, dependant on the likelihood of a given site
being subject to a wind storm and the perceived toppleproneness of a
site. Although crown lightening is a useful method of reducing the
incidence of topple, its adoption by growers will depend upon the level
of risk they are willing to accept.
Radiata pine seedlings are most prone to topple during their second or
third winter. Crown lightening needs to be applied in the autumn prior
to this critical period. The relatively low cost (10 to 20 cents/tree)
in comparison to the potentially high cost of toppling, suggests that
crown lightening can be a cost-effective operation for forest owners
who want to reduce the incidence of topple and subsequent stem
sinuosity and sweep on topple-prone sites.
(top)
This article by Peter Davies-Colley of The Tree People Ltd, and James
Turner of Forest Research,
appeared with photographs and tables in the
November 1999 issue of the New Zealand Tree Grower.
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